1. Field of the Invention
The invention relates to a hybrid vehicle that includes an internal combustion engine having an exhaust gas purification catalyst in an exhaust passage, an electric motor, a generator and an electrical storage device, and a control method for the hybrid vehicle.
2. Description of Related Art
A hybrid vehicle includes an internal combustion engine (hereinafter, simply referred to as “engine”) and an electric motor as driving sources that generate driving force for propelling the vehicle. That is, the hybrid vehicle travels by transmitting torque generated by at least one of the engine and the electric motor to a drive shaft connected to drive wheels of the vehicle.
On the other hand, in the hybrid vehicle as well, as well as a vehicle that includes only an ordinary engine as a driving source, an exhaust gas purification catalyst is arranged in an exhaust passage of the engine. The exhaust gas purification catalyst is, for example, a three-way catalyst, and is, hereinafter, simply referred to as “catalyst”. Generally, a catalyst exhibits high exhaust gas purification performance when the temperature of the catalyst (catalyst bed temperature) is higher than or equal to a predetermined activating temperature. Thus, when the temperature of the catalyst is low at the time of a start of operation of the hybrid vehicle (that is, at the time of a system start-up), or the like, the engine is started and an exhaust gas temperature is increased by retarding an ignition timing with respect to a “normal ignition timing”. By so doing, “catalyst warm-up operation” for early increasing the temperature of the catalyst is performed. Note that the “normal ignition timing” is an ignition timing referred to as “reference ignition timing, optimal ignition timing, basic ignition timing, or the like”, and is set to an ignition timing at which a combustion state is good and the efficiency of the engine is high.
On the other hand, the electrical storage device included in the hybrid vehicle is appropriately charged with regenerative energy and the power of the engine during operation of the hybrid vehicle. Thus, the remaining level of the electrical storage device is kept at an appropriate value. The remaining level of the electrical storage device is, for example, indicated by a parameter that indicates a state of charge (SOC).
Incidentally, for example, when the hybrid vehicle is not operated for an extended period of time and, therefore, the electrical storage device has been subjected to self-discharge, when the vehicle has been left standing for an extended period of time in neutral position after a system start-up of the vehicle, or the like, the remaining level of the electrical storage device can remarkably decrease. In such a case, the hybrid vehicle starts the engine to drive the generator. By so doing, the electrical storage device is charged. Operation that performs such charging is also referred to as “forced charging operation” (for example, see Japanese Patent Application Publication No. 2008-238965 (JP 2008-238965 A)).
Incidentally, in forced charging operation, for example, it is required to more quickly charge the electrical storage device as the remaining level of the electrical storage device decreases, so it is desirable to increase the amount of electric power generated by the generator by increasing the power of the engine as the remaining level decreases. As a result, during forced charging operation, a load of the engine increases as the remaining level of the electrical storage device decreases.
On the other hand, when the system is started up in the case where the catalyst temperature is low and the remaining level of the electrical storage device is extremely low, it is desirable to perform both catalyst warm-up operation and forced charging operation. However, it has been found that, when these two operations are simply performed at the same time, torque fluctuations of the engine increase due to a retarded ignition timing, particularly, in the case where the load of the engine increases through forced charging operation, torsional resonance occurs in a power transmission system due to the torque fluctuations and, as a result, hunting of an engine rotation speed occurs, and large noise (tooth hammer sound) may occur due to a collision of gears in a power transmission mechanism.